EXTRAPOLATION

by Mark Twain

The last line of this excerpt from Life on the Mississippi
is often quoted as an example of the dangers of extrapolation.
Here it is in context.

One of the Mississippi's oddest peculiarities is that of shortening
its length from time to time. If you will throw a long, pliant
apple-paring over your shoulder, it will pretty fairly shape itself
into an average section of the Mississippi River; that is, the nine
or ten hundred miles stretching from Cairo, Illinois, southward to
New Orleans, the same being wonderfully crooked, with a brief
straight bit here and there at wide intervals. The two-hundred-mile
stretch from Cairo northward to St. Louis is by no means so
crooked, that being a rocky country which the river cannot cut
much.

The water cuts the alluvial banks of the `lower' river into deep
horseshoe curves; so deep, indeed, that in some places if you were
to get ashore at one extremity of the horseshoe and walk across the
neck, half or three quarters of a mile, you could sit down and rest
a couple of hours while your steamer was coming around the long
elbow, at a speed of ten miles an hour, to take you aboard again.
When the river is rising fast, some scoundrel whose plantation is
back in the country, and therefore of inferior value, has only to
watch his chance, cut a little gutter across the narrow neck of
land some dark night, and turn the water into it, and in a
wonderfully short time a miracle has happened: to wit, the whole
Mississippi has taken possession of that little ditch, and placed
the countryman's plantation on its bank.

Pray observe some of the effects of this ditching business. The
Mississippi between Cairo and New Orleans was twelve hundred and
fifteen miles long one hundred and seventy-six years ago. It was
eleven hundred and eighty after the cut-off of 1722. It was one
thousand and forty after the American Bend cut-off. It has lost
sixty-seven miles since. Consequently its length is only nine
hundred and seventy-three miles at present.

Now, if I wanted to be one of those ponderous scientific people,
and `let on' to prove what had occurred in the remote past by what
had occurred in a given time in the recent past, or what will occur
in the far future by what has occurred in late years, what an
opportunity is here! Geology never had such a chance, nor such
exact data to argue from! Nor `development of species', either!
Glacial epochs are great things, but they are vague--vague. Please
observe. In the space of one hundred and seventy-six years the
Lower Mississippi has shortened itself two hundred and forty-two
miles. This is an average of a trifle over one mile and a third per
year. Therefore, any calm person, who is not blind or idiotic, can
see that in the Old Oolitic Silurian Period, just a million years
ago next November, the Lower Mississippi River was upward of one
million three hundred thousand miles long, and stuck out over the
Gulf of Mexico like a fishing-rod. And by the same token any person
can see that seven hundred and forty-two years from now the Lower
Mississippi will be only a mile and three-quarters long, and Cairo
and New Orleans will have joined their streets together, and be
plodding comfortably along under a single mayor and a mutual board
of aldermen. There is something fascinating about science. One gets
such wholesale returns of conjecture out of such a trifling
investment of fact.